1. Field of Invention
This invention relates to an integrated circuit (IC) fabricating process, and more particularly to a method and a system for detecting a tilt or a shift of a wafer on a hot plate in real time, and to a method and a system for monitoring a baking process of a plurality of wafers in real time based on the former method and system.
2. Description of Related Art
As the linewidth of IC process is rapidly reduced, the uniformity issue of critical dimension (CD) gets more and more important. A factor affecting the CD uniformity of a wafer layer is the thickness uniformity of the photoresist layer to be patterned into a mask defining the wafer layer, because the depth of focus is constant in the exposure.
Except the topography of the wafer surface, the levelness of the wafer on the hot plate for bake-drying the coated photoresist liquid also affects the thickness uniformity of the resulting photoresist layer, because the photoresist liquid would flow before being sufficiently dried as the wafer is tilted.
FIG. 1 depicts, in a cross-sectional view, a hot plate and a wafer placed thereon in the prior art. A typical hot plate 10 is formed with a spacer 12 and a wafer guide 14 thereon, and a sensor 16 is usually installed thereto to monitor the temperature during the baking process. As a wafer 100 that is coated with a photoresist liquid is placed surrounded by the wafer guide 14 and contacting with the spacer 12, the surface of the wafer 100 is horizontal.
However, as shown in FIG. 2A/2B, when a large enough particle 200 is present on the hot plate 10 or when the wafer 100 has been inaccurately transferred onto the same, the wafer 100 is tilted degrading the thickness uniformity of the photoresist layer. Since thickness non-uniformity of a photoresist layer cannot be detected in real time in the prior art, the CD uniformity of a wafer tilted in the photoresist baking is possibly degraded causing failure of many dies on the wafer eventually. An inaccurate transfer of a wafer to a hot plate means a shift of the wafer on the hot plate hereinafter.